The grinding of cast iron generally requires bonded abrasive articles, usually wheels, which are capable of removing large amounts of metal. Preferably, the wheels have long life for more efficient grinding and lower cost. Bonded abrasive wheels are used for trimming and cleaning of castings of cast iron; this use is commonly called snagging. In the snagging art, it has been known that bonded abrasive grinding wheels employing aluminum oxide as the sole abrasive have not been favored. It is believed that grains of standard fused aluminum oxide are too resistant to wear or breakage and, in use, the grains tend to become dull instead of breaking to expose new grinding surfaces. The dull surface of the grain results in low metal removal rates and generation of excessive frictional heat.
Therefore, in grinding, i.e., snagging of cast iron, the preferred abrasive historically has been a combination of silicon carbide and aluminum oxide. It is believed that the silicon carbide, being more friable than aluminum oxide, causes the wheel to wear at an accelerated rate and constantly expose new, sharper grinding surfaces. A more recent development has been the use of a co-fused zirconia-alumina abrasive, as described in U.S. Pat. No. 3,181,939. This material employs a particular crystalline structure to produce an even rate of wear and creation of new grinding surfaces.
While the zirconia abrasives which comprise a combination of alumina with about 35 to 50% zirconia have performed satisfactorily, their high cost has created a desire for a less expensive abrasive Further, alumina zirconia abrasives are more dense than other abrasives and there would be an advantage in a lighter abrasive which would provide less rotational stress in abrasive wheels and less operator fatigue during hand grinding.
Recently in European Patent Publication No. 0 024 099, published Feb. 25, 1981 a sol-gel alumina abrasive grain is disclosed. This grain is suggested for use in bonded abrasives, but examples and tests all involve use with coated abrasives.
U.S. patent application Ser. No. 267,495, filed May 27, 1981, now abandoned, in the names of Robert Seider and Alvin Gerk also discloses sol-gel abrasives, and suggests use in either coated or bonded applications.